Wireless communication systems such as cellular networks or wireless local area networks (WLANs) include multiple wireless communication devices that communicate over wireless channels. When operating in an infrastructure mode, a wireless communication device called an access point (AP) provides other wireless communication devices, referred to as client stations (STAs), connectivity with a network such as the Internet. Various examples of wireless communication devices include mobile phones, smart phones, wireless routers, wireless hubs. In some cases, wireless communication electronics are integrated with data processing equipment such as laptops, personal digital assistants, and computers.
Wireless communication systems can use one or more wireless communication technologies such as orthogonal frequency division multiplexing (OFDM) or code division multiple access (CDMA). In an OFDM based wireless communication system, a data stream is split into multiple data substreams. Such data substreams are sent over different OFDM subcarriers, which can be referred to as tones or frequency tones. Some wireless communication systems use a single-in-single-out (SISO) communication approach, where each wireless communication device uses a single antenna. Other wireless communication systems use a multiple-in-multiple-out (MIMO) communication approach, where a wireless communication device uses multiple transmit antennas and multiple receive antennas. WLANs such as those based on an Institute of Electrical and Electronics Engineers (IEEE) wireless communications standards, e.g., 802.11a, 802.11n, 802.11ac, can use OFDM to transmit and receive signals. Moreover, WLANs, such as ones based on the IEEE 802.11n or 802.11ac standards, can use OFDM and MIMO.
When multiple antennas are available at a transmitter, the transmitter can use a MIMO technique called beamforming to apply efficient beam patterns to the antennas to better exploit spatial selectivity and improve aggregate communication throughput. Beamforming creates a spatial gain pattern having one or more high gain lobes or beams, as compared to the gain obtained by a single omni-directional antenna, in one or more particular directions, with reduced gain in other directions. If the gain pattern for multiple transmit antennas, for example, is configured to produce a high gain lobe in the direction of a receiver, better transmission reliability can be obtained over that obtained with an omni-directional transmission. In addition to providing better link reliability, beamforming can greatly reduce the amount of power dissipated by transmitting devices. More specifically, beamforming allows a transmitting device to focus the transmission power in a particular direction when transmitting data to one or several receiving devices.